1. Field of the Invention
The present invention relates to a duct, which can be used as the intake pipe of an internal combustion engine, for example.
2. Related Art
The muffler duct is used as an intake pipe or an exhaust pipe of an internal combustion engine or an air-conditioning apparatus. This muffler duct reduces the propagation sounds to propagate from a noise source such as the internal combustion engine or the air-conditioning apparatus, or the noises which are caused by its own intake pulsations. As the known muffler duct (as referred to JP-A-5-163925, for example), a muffling device (e.g., the so-called “Helmholtz resonator”), which is composed of a box-shaped resonance box and a cylindrical member (or a communicating tube), is connected to a cylindrical duct body.
In the muffler duct of this kind, the muffling device resonates with a predetermined frequency component (or a resonance frequency F) of the noises thereby to lower the sound pressure level in the resonance frequency F. It is known that the resonance frequency F is expressed by the following Formula 1.F=(C/2π){πr2/V(L+1.6r)}1/2.  [Formula 1]
Here: C indicates a sound velocity (cm/second); r the radius (cm) of a communicating tube; V a volume (cm3) of a resonance box; and L a length (cm) of the communicating tube.
Here, it is widely known that mainly the noises of the low frequency make the user feel uncomfortable. It is, therefore, thought that the uncomfortable feel to be given to the user can be drastically reduced if the muffling device is designed to lower the resonance frequency F (or to make a low frequency). In order to lower the resonance frequency F, at least one of the volume V of the resonance box and the length L of the communicating tube may be enlarged according to the Formula 1. If the length L of the communicating tube is enlarged, however, the muffling device is enlarged to increase the bulk of the muffler duct. This raises a problem to make it difficult to mount the muffler duct on a vehicle or the like. If the volume of the resonance box is increased, on the other hand, an anti-resonance (or an anti-resonance) phenomenon may raise the sound pressure level in a specific frequency component.
JP-A-5-163925 has introduced the technique, in which the resonance frequency F is tuned by forming a plurality of openings in the partition wall of the resonance box. According to this technique, however, it is possible to lower only the noises of a frequency near the resonance frequency F but not the noises of the low frequency over a wide range.